I thought going into this post you meant a show being carried by one actor/actress, as in a bad show that was OK/good cause of a particular actess.
Cause my answer would be Heidi Blickenstaff in Little Mermaid, she saved that show for me..(And I never was a Blickenstaff fan, I never even seen TOS).
But as far as actors literally being on stage I think their are a few
Elephaba in wicked is one. Shes almost in every scene and sings 90% of the songs.
Princeton/Rod in Avenue Q. One actor plays both roles and both characters are big parts of the show. If princeton isint on stage on scene, rod is.
Theirs lots of roles like that
Shrek
Billy Elliot
either Phantom or Christine.
"Everything in life, is only for now"
-Avenue Q